Problem: 5 erasers cost $9.85. Which equation would help determine the cost of 9 erasers?
Explanation: There are several equations that could help determine the cost, each with a slightly different approach. We know the cost of 5 erasers. We want to know the cost of 9 erasers. We can write the numbers of erasers as a proportion: $\dfrac{5}{9}$ We know 5 erasers costs $9.85. We can let $x$ represent the unknown cost of 9 erasers. The proportion of these costs can be expressed as: $\dfrac{\$9.85}{x}$ The cost changes along with the number of erasers purchased, and so the two proportions are equivalent. $\dfrac{5}{9} = \dfrac{\$9.85}{x}$